It is estimated that one person a day dies in custody in the UK.

A small number are the victims of police brutality, but the majority die when prisoners are left on their own.

More than half die as a result of drugs.

People in custody are usually checked every hour and those deemed at risk of suicide or self harm are checked every half hour.

But even half an hour intervals can prove fatal.

According to the BBC One's Tomorrow's World programme, police in Hertfordshire are trying out a new alarm system which could cut deaths.

It alerts police within 30 seconds of a person stopping breathing.

Microwaves

The device includes three microwave transmitters. They are fitted to a cell wall and to the cell bed.

The receiver unit has three lights

They send out a low level of non-harmful microwaves which bounce off the walls of the cell and anyone in it.

They are sensitive enough to pick up any movement, including breathing.

The sensors are connected to a computer. It displays activity in the form of a moving line.

If the line goes flat, it means the person has either stopped breathing or left the room.

The computer is in turn connected to a receiver unit with three lights.

If a person stops breathing, an amber light and alarm comes on after 12 seconds.

After another 12 seconds, a red light flashes and the alarm sounds.

Sergeant Mark Ewan says it is much more sensitive than a video camera.

"A video camera cannot distinguish between a person lying down and not breathing and a person lying down and breathing," he said.

Vital seconds

Tina Bennett knows only too well how vital the difference between a matter of seconds and half an hour can be.

Her 59-year-old mentally ill uncle died in a police cell last January.

He had been picked up on suspicion of being drunk and disorderly.

Three powerful sensors are fitted in the police cell

Police checked him every half hour and he gave a response every time.

But when they came to release him, they found that he was dead.

The post-mortem showed that he was not drunk, but had died from pneumonia.

Ms Bennett said: "There is no worse place for a person to die than in a police cell.

"The device is a really good idea if it saves lives. If it had been there when my uncle was in custody he might even be alive today."